Human locomotion, such as walking and running, is commonly described in terms of gait. Gait is a cyclical pattern of leg and foot movement that creates locomotion. A gait cycle is defined for a single leg and begins with the initial contact of the foot with the ground or heel strike. The conclusion of a gait cycle occurs when the same foot makes a second heel strike. The gait cycle can be divided into two phases, stance phase and swing phase. Stance phase begins with heel strike and ends when the toe of the same foot leaves the ground. Swing phase begins when the foot leaves contact with the ground and ends with heel strike of the same foot.
Prosthetic and orthotic devices help restore mobility to people who lack able-bodied motion or gait. Prosthetic devices are intended to replace the function or appearance of a missing limb and can return mobility to the wearer or user. Orthotic devices are intended to support or supplement an existing limb, by assisting with movement, reducing weight-bearing loads on the body, reducing pain, and controlling or restricting movement. Prosthetic and orthotic devices are available to replace or support various portions of the body. One goal of lower limb prosthetic and orthotic devices is to help the user achieve a normal gait while reducing energy expended by the user.
Individuals have unique gait patterns. Energy or metabolic expenditure during an individual's gait depends on several factors including, body mass, stride length, step rate, and other physical and environmental factors. Individuals have physical and metabolic limits, which determine the speed and distance an individual can travel on foot. Other forces hindering human gait are air drag and rolling friction. Decreasing the metabolic cost for an individual's gait allows the individual to run faster or travel for a longer distance while minimizing the energy expended by the individual.
Over-speed running and training is a method of reducing metabolic cost for gait. Using pulley systems and springs, two runners are coupled together to reduced metabolic cost when one runner pulls another. When an external, horizontal force is added to a runner on a treadmill, the force reduces the positive horizontal ground reaction force needed for push-off. The force increases the negative horizontal ground reaction force at landing which increases braking forces. However, attaching a pulley system to an individual to assist with gait leaves the individual tethered to the treadmill or to another runner. Negative damping has also been used to create an external force in the same direction as a velocity of the body by adding positive power. However, negative damping can cause an unstable amount of energy added as velocity increases.